Some are of opinion that, as no kind of impulse of the nutritive juices is required in vegetables, but that these are attracted by the parts which require them, and flow in to take the place of what has been lost; so neither is there any necessity for an impulse in animals, the vegetative faculty in both working alike. But there is a difference between plants and animals. In animals, a constant supply of warmth is required to cherish the members, to maintain them in life by the vivifying heat, and to restore parts injured from without. It is not merely nutrition that has to be provided for. So much for the circulation; any impediment, or perversion, or excessive excitement of which, is followed by a host of dangerous diseases and remarkable symptoms: in connexion with the veins--varices, abscesses, pains, hemorrhoids, hemorrhages; in connexion with the arteries--enlargements, phlegmons, severe and lancinating pains, aneurisms, sarcoses, fluxions, sudden attacks of suffocation, asthmas, stupors, apoplexies, and innumerable other affections. But this is not the place to enter on the consideration of these; neither may I say under what circumstances and how speedily some of these diseases, that are even reputed incurable, are remedied and dispelled, as if by enchantment. I shall have much to put forth in my Medical Observations and Pathology, which, so far as I know, has as yet been observed by no one. That I may afford you still more ample satisfaction, most learned Riolanus, as you do not think there is a circulation in the vessels of the mesentery, I shall conclude by proposing the following experiment: throw a ligature around the porta close to the liver, in a living animal, which is easily done. You will forthwith perceive the veins below the ligature swelling in the same way as those of the arm when the bleeding fillet is bound above the elbow; a circumstance which will proclaim the course of the blood there. And as you still seem to think that the blood can regurgitate from the veins into the arteries by open anastomoses, let the vena cava be tied in a living animal near the divarication of the crural veins, and immediately afterwards let an artery be opened to give issue to the blood: you will soon observe the whole of the blood discharged from all the veins, that of the ascending cava among the number, with the single exception of the crural veins, which will continue full; and this certainly could not happen were there any retrograde passage for the blood from the veins to the arteries by open anastomoses. ANATOMICAL EXERCISES ON THE GENERATION OF ANIMALS; TO WHICH ARE ADDED ESSAYS ON PARTURITION; ON THE MEMBRANES, AND FLUIDS OF THE UTERUS; AND ON CONCEPTION. BY WILLIAM HARVEY, DOCTOR OF PHYSIC, AND PROFESSOR OF ANATOMY AND SURGERY IN THE COLLEGE OF PHYSICIANS OF LONDON. LONDON, 1651. To the learned and illustrious the President and Fellows of the College of Physicians of London.